It is thus already known to unite bone fragments by means of a screw whose shank is pierced over its entire length and which has an outside thread over its entire length. The screw is associated with a separate screw head that has a through internal well so as to be capable of being threaded onto the screw shank. The internal well is provided with tapping complementary to the thread of the screw shank so as to be capable of co-operating therewith and of moving along the entire length of the thread so as to come into abutment in the proximal zone of the screw shank. The screw head also has an outside thread at a smaller pitch than the thread of the screw shank.
In general, such a device gives results that are satisfactory in terms of osteosynthesis treatment since, by screwing the screw shank through the bone fragments, it enables the bone fragments to be brought together, enabling the various bone fragments to be united by jointing at the line of fracture. That device also makes it possible to put the bone fragments under a small amount of compression because of the existence of a pitch differential between the outside screw thread on the screw head and the thread on the screw shank. The existence of a screw pitch differential at the outside thread of the screw head makes it possible, when the head begins to penetrate into a portion of bone, to urge the bone fragments towards one another, which implies putting them into compression. Such compression can also be increased slightly by rotating the head of the screw on its own, thereby making it possible to control or perceptibly increase the amount of compression.
Nevertheless, such a device suffers from drawbacks that put a limit on the amount of compression that can be applied to bone fragments because of the existence of the thread on the screw shank which opposes the application of compression because it is anchored directly in the bone fragments. In addition, the presence of the thread on the screw shank can give rise to bone trauma that is harmful for proper resorption of the fracture.
Other types of dual-pitch compression osteosynthesis screw also exist in the prior art, such screws not having an independent screw head and consequently enabling only a moderate amount of compression to be applied to the bone fragments that are to be united.